Attapulgite clay (Fuller's Earth) particles naturally occur as colloidally dimensioned, high aspect ratio needle-like shaped crystals that are bundled together, and the bundles are agglomerated into clusters. As the attapulgite deposits were formed, other clay types were formed simultaneously or were introduced through air and water movements. Other mineral types also were formed in place or transported into the deposits during their formation, resulting in a variety of non-attapulgite materials being present in levels approaching 20 weight percent or more. In order for the attapulgite to be in a physical form acceptable for commercial use, it needs to be processed into a powder. The attapulgite clay is historically processed by a grinding or milling operation, followed by particle size separation and drying to a moisture content of approximately 10%. The non-attapulgite matter is also reduced in size and partially removed during the particle size classification operation. The non-clay minerals can be abrasive and must either be removed or reduced in size to lessen their abrasive characteristics in order for the attapulgite powder to be useful in end use applications.
A grinding/milling operation will partially break up the bundle clusters with the undesirable result of fracturing of individual attapulgite particles. The individual attapulgite particles provide the thixotropic, anti-settling, and/or binding properties to the end use application. Particles with higher length to width, or aspect ratios, typically are more efficient than particles with lower aspect ratios.
In end use applications, a chemical dispersant, predominantly tetrasodium pyrophosphate (TSPP), will at best, separate a majority of the attapulgite bundles into individual particles in an aqueous medium under moderate-to-high shear agitation. However, TSPP will also disperse other clay types present. Individual clay particles of each clay species present, will remain suspended in water. U.S. Pat. No. 3,569,760 has shown that non-clay minerals will not remain suspended in the water and will settle to the bottom due to gravitational forces if the clay-water slurry is low enough in viscosity. The relatively large non-clay minerals, which by their means of higher hardness, can resist size reduction and thus can also be removed by screening, centrifuging, via sedimentation tanks, hydrocyclone, or other physical separation means. The non-attapulgite clay minerals, such as montmorillonite and sepiolite, will remain mixed in with the attapulgite clay. The non-attapulgite clays do not have the same performance characteristics as attapulgite clay, some which are detrimental to performance in particular applications. At the present time, the suppliers and users of attapulgite are dependent on the quality of natural deposits for the concentration of non-attapulgite clay content. The availability of high purity attapulgite clay relies on the economically inefficient selective mining of the deposits.
Gantt et. al. in U.S. Pat. No. 5,358,120, have shown that other types of previously commercially available dispersants such as sodium polyacrylates can be used with clay. However, sodium polyacrylates with molecular weights less than 4,300, while effective on kaolins and bentonites, are typically not as effective with attapulgite when compared to TSPP, and thus not generally used.
Attapulgite clay provides thixotropic and binding properties through the process of re-flocculating after being typically dispersed at less than 5% solids in an aqueous system, whereby a gel structure is generated. Various salts, at additive levels, can act as re-flocculating agents.
Historically, attapulgite clay is dried to a free moisture content of approximately, 10-16%, with an 8 or 9% minimum. At lower moisture contents, the attapulgite begins to lose its thixotropic properties and does not readily re-disperse in water. Brooks, in U.S. Pat. No. 4,966,871, has shown that it is possible to vacuum dry attapulgite down to less than 2% and still retain its thixotropic properties, the importance being that the attapulgite is dried under vacuum conditions.
Attapulgite clay is frequently used as an additive product, often comprising only 1 to 3 weight percent of the final composition. Attapulgite clay does not disperse completely in a liquid medium at these low concentrations. The believed cause is the lack of clay to clay collisions needed to break up the agglomerates and bundles. A technique used to overcome this behavior is one in which a 20 to 25% slurry is made, and when completely dispersed, it is diluted with water down to the desired ultimate concentration.
There is a need to be able to efficiently and economically separate attapulgite clay from other clay types and from non-clay minerals.